©Nicholas Larghi/TNC Photo Contest 2021

Blue Carbon: An Important Climate Change Mitigation and Adaptation Opportunity

U.S. Nature4Climate convened a panel to discuss the challenges and opportunities surrounding “blue carbon” as a climate mitigation strategy. According to the NOAA National Ocean Service, “blue carbon” is simply the term for carbon captured by the world’s ocean and coastal ecosystems. Our panel, made up of experts from the U.S. Environmental Protection Agency, The Nature Conservancy, Environmental Defense Fund, The Pew Charitable Trusts, and Silvestrum Climate Associates walks through the climate and co-benefits of blue carbon strategies, the uncertainties around measuring the carbon sequestration potential of these practices, the research gaps and challenges facing decision-makers seeking to implement these strategies, as well as a way forward for addressing these challenges.


  • Nathan Henry – Program Manager – U.S. Nature4Climate

 Discussion Participants:

  • Steve Crooks – Principal, Wetland Science and Coastal Management – Silvestrum Climate Associates
  • Jamie Collins – Marine Biochemical Scientist – Environmental Defense Fund
  • Sylvia Troost – Senior Manager, Conserving Marine Life in the United States – The Pew Charitable Trusts
  • Phil Colarusso – Marine Biologist – U.S. Environmental Protection Agency
  • Lindsey Smart – Climate and Ocean Scientist – The Nature Conservancy
  • Adam Reilly – Physical Scientist, Watershed and Nonpoint Source – U.S. Environmental Protection Agency

What is Coastal Blue Carbon?

Decision-makers, scientists, economists, environmental organizations and coastal land stewards are increasingly recognizing the role that so-called “blue carbon” ecosystems can play in helping to mitigate climate change and adapt to its impacts. As noted by Steve Crooks, “Blue carbon recognizes the role of coastal and marine ecosystems in the global carbon cycle. If we do a better job of managing coastal ecosystems, through conservation, restoration, or some form of sustainable development, then we can have a positive benefit on the climate.”

What exactly are blue carbon ecosystems? According to Phil Colarusso, “The ecosystems traditionally recognized as coastal blue carbon include mangroves, salt marshes, and seagrasses because they sequester carbon and can hold it for decades to centuries.”

How Does Blue Carbon Help Mitigate Climate Change?

The carbon mitigation potential of blue carbon wetlands is significant. Steve Crooks noted that, once carbon sequestration and methane emissions by these wetlands are taken into account, coastal wetlands sequester about 6.5 million tons of carbon dioxide a year– equivalent to the yearly emissions of over 1.4 million cars. To avoid the worst impacts of climate change we need to both reduce transportation emissions and protect and restore the carbon sequestration potential of blue carbon.

Blue carbon projects and related policies can both help sequester carbon from the atmosphere, and avoid the emission of carbon that is already sequestered in coastal wetland environments. As noted by Jamie Collins, “Avoiding the degradation and destruction of existing coastal wetland systems is an avoided emissions pathway. By contrast, coastal restoration represents a sequestration pathway – a way to draw down and then store more carbon.”

Steve Crooks built on Jamie’s comment by emphasizing the importance of keeping existing wetlands intact:

“Because coastal wetlands have been accumulating carbon gradually for a very long period of time, then the emissions side of the curve is much steeper than the restoration side of it. If you imagine that a salt marsh might have 1,000 tons of carbon per hectare (about the size of 2.5 football fields), you can emit that very quickly – within just a few years to a decade if you convert that to some other use. But to  put back a meter of soil is going to take a hundred years or more. So conserving existing wetlands is extremely important when you think about emissions.”

Coastal Restoration Provides Many Other Benefits Besides Carbon Sequestration

In addition to sequestering carbon, panelists noted that coastal blue carbon ecosystems provide a wide range of additional environmental and economic “co-benefits.” As noted by Lindsey Smart:

“These systems have numerous co-benefits beyond climate mitigation that are important to value. Trying to find innovative ways to value some of those non-material benefits is really important, because it’s going to help with  buy-in, uptake and social acceptance of these strategies. These ecosystems provide coastal protective benefits and nursery habitat important for fisheries. They provide cultural ecosystem services – spiritual benefits and cultural heritage value. People recreate in these areas – they are beautiful and aesthetically pleasing.”

Adam Reilly highlighted that tools are available to help quantify the value of these benefits, which can help strengthen the case for taking action to restore and protect coastal ecosystems:

“There are many different ways to look at the benefits of these systems, and these types of calculations provide a more effective way for us to communicate the benefits of the system.  For example, you can apply the U.S. Government’s most recent 2021 estimate on the value of a unit of carbon to estimate the value of carbon stored in a given region. You can do the same thing with methane (or nitrous oxide) emissions that might be avoided through the restoration of a salt marsh. The more ways you allow people to understand the benefits of blue carbon preservation or restoration, the more public support you can generate, so there’s a lot of opportunity out there.”

Measuring the Climate Mitigation Benefits of Coastal Blue Carbon can be Challenging

Our panelists noted that there remain uncertainties and data gaps hindering our efforts to fully understand the climate change impacts of coastal blue carbon, and this understanding is further constrained by limited funding to support systematic research. As Lindsey Smart observed:

“The impacts of both climate change and disturbance from land use activities greatly influence current and restored blue carbon ecosystems, their carbon sequestration rates, storage, and burial rates. Getting a better understanding of that through increased monitoring is really key. But finding the funding for that monitoring can be challenging.”

Steve Crooks noted the complexity of measuring greenhouse gases (GHGs) over time, and suggested a much more robust infrastructure is necessary to accurately monitor and model blue carbon ecosystems:

“You can measure soil carbon accumulation and come back every 25 years to see how much has accumulated there. Greenhouse gas fluxes are more complicated than that. Imagine, for example, if the U.S. had only 12 weather stations for all of its modeling of weather. How good would our modeling be from that? We have thousands of weather stations across the U.S. Imagine if we had a similar network of instrumentation to measure greenhouse gases across the U.S.  What if we had a much higher density network of instrumentation around coastal systems related to greenhouse gases? How could we model things then?

Jamie Collins noted that building a resilient infrastructure for research and monitoring will require a significant investment in incentives and innovation:

“Putting the ideal sort of GHG monitoring and data collection infrastructure into place could be hugely expensive, and that kind of equipment, such as eddy covariance flux towers, is vulnerable to storm damage, etc. So it will require a significant financial incentive, or a government mandate, to ramp up measurement of greenhouse gas fluxes in coastal restoration projects to the level necessary. At the same time, we need some sort of incentive to catalyze the development of new sensors and technologies that are less expensive and less vulnerable.”

Phil Colarusso added that measuring carbon dioxide is only one part of the puzzle – other greenhouse gases also need to be considered:

“We should be focusing on more than just carbon – we also need to understand fluxes of other greenhouse gases like nitrous oxide, which is a much more powerful, potent greenhouse gas than carbon dioxide. There is only one publication and one study that I’m aware of that looks at this issue, and they provide conflicting results. One suggests that seagrasses are a source of nitrous oxide and the other says they are a sink. Getting a handle on this is critical if we’re talking about climate.”

Colarusso also highlighted that measuring the long-term carbon sequestration of these projects is also challenging, noting “For the validation, you need to somehow guarantee that the carbon is going to stay there for 100 years. I know I’m not going to be around in 100 years measuring carbon in seagrass meadows.”

The uncertainties around blue carbon’s climate mitigation impacts have significant impacts on real-world decision-making, both in the corporate and policy space. As Adam Reilly observed:

“So many private companies are looking to build up their green bona fides and purchasing offset credits seems to be an attractive way for them to do that. With blue carbon, there is an opportunity there, but there is still not enough validation in a lot of these crediting programs to ensure that the offsets they are claiming are actually offsetting real fossil fuel emissions. It’s an exciting idea, but I don’t know that we’re there yet to do it at any type of scale. Restoration project validation is key.”

© George Steinmetz/TNC

Addressing Uncertainties & Spurring Action

Panelists suggested several factors that could help stimulate research to help fill research gaps and address uncertainties. Building blue carbon into regulatory processes is key.  For example, Sylvia Troost suggests:

“From a regulatory standpoint, if decision makers make it a priority to recognize this ecosystem service by accounting for blue carbon in coastal land use planning, permitting and other regulatory processes,  that would certainly jumpstart more attention being paid to research and monitoring.

It’s exciting to see states like Maine, New Jersey, Maryland, Louisiana, California and Oregon incorporating coastal blue carbon ecosystems into their natural and working lands strategies for climate mitigation. Explicitly recognizing this important ecosystem service provided by coastal wetlands can help drive efforts to better quantify, conserve and restore blue carbon habitats.”

Troost also noted that implementation of the recently passed Bipartisan Infrastructure Investment and Jobs Act could help spur action:

“Generally speaking, many of the same strategies related to climate resilience that are eligible for funding under the Infrastructure Bill could help conserve and expand blue carbon ecosystems, enhancing carbon sequestration and storage; for example, expanding the use of living shorelines to protect coastal communities or removing dams and other barriers that have restricted vital sediment flows to downstream coastal wetlands.”   

Lindsey Smart highlighted steps being taken to better account for the multiple environmental benefits provided by blue carbon projects:

“The Nature Conservancy is collaborating with leaders in the carbon market and resilience modeling fields to develop a Blue Carbon Resilience Credit market mechanism, which is an approach that stacks carbon offsets from climate mitigation benefits with coastal resilience credits derived from flood risk reduction benefits these habitats provide. It is an innovative way to capture the multiple values blue carbon ecosystems provide.”

The panel also acknowledged a need to account for equity when planning blue carbon projects, which is especially important in light of recent research indicating the disproportionate risks that underserved and/or majority minority communities face due to rising sea levels. Lindsey Smart highlighted the importance of ensuring that the benefits of blue carbon projects are equitably distributed: 

“Considering equity dimensions with all of these projects is also something really important to consider. This extends beyond the technical aspects of monitoring, verification, and permanence that we’ve been talking about. Recent concerns that climate mitigation activities may undermine local equity, stem from inequities witnessed in REDD+ projects. Critiques highlight that these activities may result in adverse impacts to local community access rights, livelihoods, food security and overall health and well-being.  It is imperative that these activities are part of an inclusive process that considers equity from the onset, to ensure equitable distribution of both costs and benefits.”

Emerging Blue Carbon Strategies – Oceanic Blue Carbon

While much of our discussion focused on coastal blue carbon strategies that are already underway in the United States, the panel also spent some time discussing emerging open-ocean blue carbon strategies that deserve more study. Jamie Collins noted that the Environmental Defense Fund had convened a working group of 35 government and academic scientists, including social scientists and economists, to explore open-ocean pathways. Some of the strategies, which have significant potential as natural climate solutions but are surrounded by large uncertainties that limit their current suitability as sources of high-quality carbon credits, include:

  • Rebuilding Whale & Fish Populations:  Sequestering carbon by rebuilding great whale population numbers.There is some evidence carbon can be sequestered in the biomass of whale carcasses through whale fall – that is, the sinking of whales to the bottom of the ocean when they die. There is also a much more scientifically speculative indirect sequestration pathway based upon the recycling of nutrients in whale feces. Another potential opportunity is to rebuild commercially valuable fish populations based on their carbon value.
  • Ocean Afforestation:  Growing seaweeds in areas where they haven’t traditionally grown, such as the open ocean, due to the lack of natural substrates seaweeds need to anchor themselves. By providing artificial substrate to create anchors, and supplying the appropriate nutrients if they do not exist naturally in the surrounding waters, we can stimulate seaweed growth. These seaweeds fix carbon as they grow and then release some of this carbon in dissolved forms that resist degradation by bacteria. Some of the remaining biomass could be sunk as a sequestration strategy, or used in other pathways back on land that can store carbon or reduce emissions. 
  • Bottom Trawling & Fishing Restrictions: Bottom-trawling of the open ocean for fish can disturb substantial amounts of carbon stored in oceanic sediments. Some of this disturbed carbon — we don’t at present really understand how much — could make it back to the surface ocean, where it can be released back to the atmosphere as CO2. Restricting bottom trawling in Arctic areas newly available to fishing due to retreating sea ice could represent a significant avoided emission. Separately, preventing new fishing on mesopelagic fish species that inhabit the bottom of the sunlit layer of the ocean could also help prevent new carbon emissions. These species transfer a huge amount of carbon from the surface ocean to the deep ocean every year. While fish species in this layer of the ocean are not currently being fished commercially due to logistical challenges, that is likely to change in future years.

Steve Crooks highlighted some of the challenges around incorporating oceanic blue carbon into climate mitigation strategies:

“The science has been developing around some of these strategies. We know that kelp, for instance, is a positive source of carbon sequestration. The challenge is quantifying it. Can we develop monitoring frameworks around it? Are the potential sinks significant enough to even bother thinking about it? There are regulatory challenges as well. If it’s all in the open ocean, who is responsible for maintaining safeguards, etc.

Lindsey Smart added that monitoring these ecosystems can be challenging since the stored carbon resulting from ocean afforestation strategies can end up being stored in a location far away from the actual kelp. 

However, Sylvia Troost noted that, like the coastal blue carbon strategies discussed earlier, oceanic blue carbon strategies provide other environmental benefits, making them a no-regrets strategy even if the climate mitigation benefits are poorly understood.

“From  a policy standpoint, similar to what we were talking about with coastal blue carbon, many of the tools used to protect biodiversity in the oceans such as marine protected areas and seafloor habitat protections, intuitively would have benefits in ensuring oceanic carbon stores remain intact from human disturbances.”

Lindsey Smart agreed, suggesting that even if these strategies are implemented to achieve other conservation priorities, like marine habitat protection, they can offer climate mitigation benefits as well. As gaps in the science around oceanic blue carbon are being addressed, these marine protection strategies are well-positioned to provide multiple environmental benefits in the near term.  

Steve Crooks closed the discussion by noting that these ocean protection strategies are win-win solutions –  “If you create marine protected areas, you can create more fish, overall, so you can actually protect seabed carbon. You can have more fish at the same time, so everybody wins.”

This expert discussion of blue carbon took place in February 2022.